1. Field of the Invention
The present invention generally relates to a monocrystalline silicon wafer, and more particularly, to a monocrystalline silicon wafer improved so that enhanced oxidation will not be caused at the time of forming an oxide film on a surface thereof. The present invention also relates to a method of thermally oxidizing a surface of such a monocrystalline silicon wafer.
2. Description of the Background Art
Formation of a thermal oxide film on a surface of a monocrystalline silicon wafer is an essential step in a method of manufacturing a semiconductor device. FIG. 11 is a series of cross sections sequentially showing a semiconductor device in each of the steps in a method of manufacturing a field effect transistor, including the step of forming a silicon oxide film, that is, a thermal oxide film.
Referring to FIG. 11(a), a monocrystalline silicon substrate 1 is prepared.
Referring to FIG. 11(b), a LOCOS (LOCal Oxidation of Silicon) oxide film 2 for separating an active region from another is formed at a main surface of monocrystalline silicon substrate 1. Then, the surface of monocrystalline silicon substrate 1 is thermally oxidized, whereby a silicon oxide film 3 which is a base of a gate oxide film is formed. A polysilicon film 4a which is a base of a gate electrode is formed to cover silicon oxide film 3.
Referring to FIG. 11(b) and (c), a gate insulating film 5a has a gate electrode 6a are formed on an active region by selective etching of silicon oxide film 3 and polysilicon film 4a.
Referring to FIG. 11(c) and (d), impurity ions 30 are implanted into a surface of the active region using gate electrode 6 has a mask, thereby forming a source/drain region 7.
Referring to FIG. 11(d) and (e), a interlayer insulating film 8 is formed to cover gate electrode 6. A contact hole 9 for exposing a portion of a surface of source/drain region 7 is formed in interlayer insulating film 8. An aluminum (Al) interconnection 31 which is a bit line is formed to be electrically in contact with source/drain region 7 through contact hole 9, whereby an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is completed.
FIG. 12 is a diagram conceptually showing a thermal oxidation furnace (vertical type furnace) for carrying out the step of oxidizing a surface of a semiconductor substrate to form a silicon oxide film (the step shown in FIG. 11(b)).
Referring to FIG. 12, the thermal oxidation furnace is provided with a reaction tube 10. Reaction tube 10 is surrounded by an outer wall 11. A heater 12 is provided around outer wall 11. A gas discharging tube 13 for discharging gas within reaction tube 10 is connection to outer wall 11. A gas supplying tube 14 for supplying reactive gas into reaction tube 10 is connected to reaction tube 10. A boat 32 is to be inserted into reaction tube 10 by a boat elevator 15. Dummy monitor wafers 16 and wafers 17 for practical use are to be placed in boat 32. Dummy monitor wafers 16 are located in reaction tube 10 so as to control gas flow and to prevent maldistribution of temperature. Normally, 26 dummy wafers 16 and 100 wafers 17 for practical use are placed therein.
An LOCOS oxide film is also formed using this thermal oxidation furnace.
FIG. 13 is a series of cross sections sequentially showing a semiconductor device in each of the steps in a method of forming an LOCOS oxide film.
Referring to FIG. 13(a), a monocrystalline silicon substrate 1 is formed.
Referring to FIG. 13(b), an SiO.sub.2 film 18 is formed on monocrystalline silicon substrate 1. A silicon nitride film 19 is then formed on SiO.sub.2 film 18 to cover a portion of the substrate where an active region is to be formed.
Referring to FIG. 13(b) and (c), a surface of monocrystalline silicon substrate 1 is oxidized using silicon nitride 19 as a mask, thereby forming LOCOS oxide film 2.
Referring to FIG. 13(c) and (d), silicon nitride film 19 is removed, and LOCOS oxide film 2 for separating an active region from another is formed at a main surface of semiconductor substrate 1.
Formation of LOCOS oxide film 12 shown in FIG. 13(c) is carried out by the thermal oxidation furnace shown in FIG. 12.
As described above, a silicon oxide film is formed by inserting a monocrystalline silicon wafer into a thermal oxidation furnace and thermally oxidizing the wafer. In particular, a gate oxide film is one of the important elements in a silicon oxide film which determine the property of a semiconductor device. Accordingly, thickness of a gate oxide film has been managed very strictly. Nevertheless, variation in the thickness of the gate oxide film has not been eliminated.